I choose water because you'd be able to survive forever. Bonus you get to control peoples bodies. Then again energy bending would be pretty cool too, because you could be all like "no bending for you" while shaking your finger like Dikembe Mutombo.

probably fire, since i love how destructive it can be. I would like the feeling of holding one of the most destructive elements in my hands and then using it to defeat an enemy or obstacle that would other wise be impassable if it wasn't for my fire bending